Blends of rubber and plastic have been produced with the hope of making thermoplastic elastomers, which are compositions that exhibit at least some of the properties of thermoset elastomers and yet are processable as thermoplastics. For example, U.S. Pat. No. 3,806,558 teaches blends of monoolefin copolymer rubber and polyolefin resin, where the rubber is partially cured under dynamic conditions. Peroxide curatives may be employed in this process together with auxiliary substances such as sulfur, maleimides including bismaleimides, polyunsaturated compounds (e.g., cyanurate), and acrylic esters (e.g., trimethylolpropanetrimethacrylate). The gel content of the rubber within these blends does not exceed 96%.
U.S. Pat. No. 4,247,652 teaches blends of a peroxide-curable olefin copolymer rubber. (e.g., EPDM), a peroxide-decomposing olefin plastic (e.g., isotactic polypropylene), a peroxide-non-curable hydrocarbon rubbery material (e.g., butyl rubber), and a mineral oil softener. These blends are dynamically cured by employing a peroxide in combination with a peroxy-curing promoter such as sulfur, p-quinone dioxime, p,p′-dibenzoyl- quinone dioxime, N-methyl-N,4-dinitrosoaniline, nitrobenzene, diphenyl guanidine, trimethylol propane-N,N′-m-phenylene dimaleimide, or a polyfunctional vinyl monomer such as divinyl benzene or triallyl cyanurate, or a polyfunctional methacrylate monomer such as ethyleneglycol dimethacrylate, diethyleneglycol dimethacrylate, polyethyleneglycol dimethacrylate, trimethylol propane trimethacrylate or allyl methacrylate. Despite the fact that this patent teaches partially-cured rubber, the rubber can purportedly be cured in the range from about 20 to 99% gel content in cyclohexane at 35° C.; although the data set forth in the specification suggests cured levels much lower than 99%.
Similarly, U.S. Pat. No. 4,785,045 teaches dynamically cured blends of a peroxide-crosslinkable olefinic copolymer rubber, a peroxide-crosslinkable polyolefin resin, and a peroxide-decomposable polyolefin resin. The dynamic cure is effected with a peroxide in conjunction with a crosslinking aide such as p-quinonedioxime, p,p′-dibenzoyl quinonedioxime, N-methyl-N,4-dinitrosoaniline, nitrobenzene, diphenyl guanidine, trimethylolpropane-N,N′-m-phenylene dimaleimide, divinylbenzene, triallyl cyanurate, ethylene glycol dimethacrylate, diethylene glycol dimethacrylate, polyethylene glycol dimethacrylate, trimethylolpropane trimethacrylate, allyl methacrylate, vinyl butyrate and vinyl stearate. Despite the fact that this patent teaches partially-cured rubber, the rubber can purportedly be cured in the range from about 45 to about 98% in cyclohexane. Polyfunctional methacrylate compounds that are useful as cross-linking aids in the manufacture of rubber and plastic blends are also disclosed in U.S. Pat. No. 6,765,052, which discloses ethylene glycol dimethacrylate, polyethylene glycol dimethacrylate, trimethylolpropane trimethacrylate and allyl methacrylate.
With the desire to improve upon the melt strength of blends of rubber and plastic where the rubber is partially cured, U.S. Pat. No. 6,646,056 teaches the use of multi-functional monomers with or without the presence of free radical initiators. These multi-functional monomers must include acrylate functional monomers, which are preferred over methacrylate monomers.
Blends of rubber and plastic where the rubber is fully cured are also disclosed. For Example, U.S. Pat. No. 4,130,535 teaches thermoplastic vulcanizates comprising blends of olefin rubber and thermoplastic olefin resin in which the rubber is completely cured (i.e., no more than 3% of the rubber is extractable in cyclohexane at 23° C.). Numerous cure systems are disclosed including those based upon sulfur or peroxides.
Recognizing that the use of peroxides to fully cure thermoplastic vulcanizates can have an undesirable side effect on the plastic, U.S. Pat. No. 5,656,693 teaches the use of elastomeric copolymer rubber deriving from the copolymerization of ethylene, an α-olefin, and 5-vinyl-2-norbornene. When using this particular rubber, peroxide-cured thermoplastic vulcanizates having a high degree of cure could be achieved with the use of less peroxide than had been used in the past. By employing lower levels of peroxide, those physical properties attributable to the plastic phase could be maintained.
Inasmuch as the use of peroxide cure systems to dynamically cure—and ideally fully cure—the rubber phase of thermoplastic vulcanizates may offer many advantages, there remains a desire to improve upon the peroxide cure system, particularly with regard to the impact that these systems have on the plastic phase of the thermoplastic vulcanizates.